Treatise on the Sun

Time is said to be of two kinds: the first is "gross" or measurable (Martta), and the second is "subtle" or immeasurable (Amurtta). (10)
original: "मर्त्तश्चामूर्त्त उच्यते" — This distinction separates the time we can perceive and calculate from the infinitesimal or infinite time that lies beyond human measurement.

The measurable time is that which begins with the breath (Prana), while the immeasurable begins with the "atom" (Truti). Six breaths are said to make one Vinadi; sixty of those Vinadis are known as a Nadika. (11)
A Prana (breath) is roughly 4 seconds. A Vinadi is 24 seconds, and a Nadika (or Ghati) is 24 minutes.

Sixty Nadikas constitute a "starry" day and night (Nakshatra Ahoratra). Thirty of these days make a month. A "civil" month (Savana) is likewise defined by thirty sunrises. (12)
A "starry" or sidereal day is the time it takes for the earth to rotate relative to the distant stars, rather than the sun.

A lunar month (Aindava) is measured by lunar days (Tithis), while a solar month (Saura) is determined by the Sun’s entry into a zodiac sign (Samkranti). Twelve months are called a year; this is also known as a "Day of the Gods." (13)

The Gods and the Demons (Asuras) live at the North and South poles respectively; therefore, their day and night are the reverse of one another. Six times sixty (360) of these divine days make one year of the Gods. (14)

Twelve thousand of these divine years are called a "Cycle of Four Ages" (Caturyuga). In the count of solar years, this consists of four million, three hundred and twenty thousand years. (15) 4,320,000
original: "चतुर्युगमुदाहृतं" — This refers to the Mahayuga, the great cycle encompassing the Krita, Treta, Dvapara, and Kali ages.

The "twilights" and "dawn-junctions" (Sandhya and Sandhyamsha) must be included in this Caturyuga. The arrangement of the four ages—the Krita and the others—is determined by the four quarters of Virtue (Dharma). (16)
Virtue is imagined as a bull standing on four legs in the first age, losing one leg in each subsequent age.

The tenth part of a Caturyuga, multiplied by four, three, two, and one, gives the length of the Krita, Treta, Dvapara, and Kali ages respectively. A sixth part of each age constitutes its own dawn and twilight junctions. (17)

Seventy-one cycles of these ages are called a Manvantara (the age of a Manu). At its conclusion, a transition period (Sandhi) is declared, which is the length of a Krita age and takes the form of a Great Flood (Jalaplaba). (18)
original: "जलप्नब" — This refers to the periodic dissolution or deluge that cleanses the world between the reigns of different Manus.

In one Kalpa (an Aeon), there are fourteen such Manus along with their transition periods. At the very beginning of the Kalpa, there is a fifteenth transition period, which is also the length of a Krita age. (19)

Thus, a Kalpa is made of one thousand cycles of four ages; it is the force that brings about the destruction of all beings. This Kalpa is called a "Day of Brahma." His Night is said to be of the same duration. (20)

His total lifespan is one hundred years, calculated by these days and nights. One half of his life has already passed; of the remaining half, this is the first Kalpa. (21)
The text places the current moment in the "Varaha Kalpa," the first day of the 51st year of Brahma's life.

Starting from this Kalpa, the Manus are:

1\. Svayambhuva, 2. Svarocisha, 3. Uttamaja, 4. Tamasa, 5. Raivata, 6. Chakshusha, 7. Vaivasvata The current Manu of our age., 8. Arka-savarni, 9. Daksha-savarni, 10. Brahma-savarni, 11. Dharma-savarni, 12. Rudra-savarni, 13. Deva-savarni, 14. Indra-savarni.